The technical field of the present invention is prosthetic vascular materials, and more specifically is biocompatible, thromboresistant substances and methods of their preparation.
Exposure of blood to artificial surfaces usually leads to deposition of a layer of adherent platelets, accompanied by activation of the intrinsic coagulation system, and ultimately to the formation of a thrombus. In fact, significant blood/materials interaction can occur on a single pass through a prosthetic arterial graft. The types of blood proteins initially adsorbed or bound to synthetic surfaces may include proteins involved in contact coagulation. Contact coagulation or the extrinsic pathway of coagulation is a complex pathway of biochemical events that induces fibrin formation, platelet and complement activation, chemotaxis, kinin generation, and activation of fibrinolytic components. In addition, each of these events augments subsequent biochemical pathways often controlled by positive and negative feedback loops. Thus, thrombosis induced by contact with artificial materials is a major obstacle in the development and use of internal prostheses and extracorporeal devices such as artificial vessels and organs, and cardiopulmonary bypass and hemodialysis equipment.
Materials having varying degrees of thromboresistance have been utilized in vascular prostheses with limited success. These materials include corroding (self-cleaning) metals, synthetic polymers such as polydimethyl siloxane, Teflon, acylates and methacrylates such as Dacron, electrets, anionic copolymers, and hydrogels for a review see Salzman et al. (1987) in Hemostasis and Thrombosis, Basic Principles and Clinical Practice (Colman et al., eds.) J. B. Lippincott Co., Phila., Pa., pp. 1335-1347).
To decrease the chances of thrombosis due to extended periods of contact with such artificial materials, patients have been treated with systemically administered anti-coagulant, anti-platelet, and thrombolytic drugs. These include any compound which selectively inhibits thromboxane synthetase without affecting prostacycline synthetase, affects platelet adherence as well as aggregation and release, enhances vascular PGI2 production, and/or inhibits both thrombin- and thromboxane-mediated platelet aggregation. Such compounds include aspirin, sulfinpyrazone, dipyridamole, ticlopidine, and suloctidil. However, treatment with these drugs often elicits unwanted side effects including systemic hemmorhaging and the inability to initiate and complete desired clotting elsewhere in the body.
To improve on the thromboresistance of artificial materials, biologically active molecules having thrombolytic, anticoagulating, thrombogenesis-inhibiting, and/or platelet inhibiting abilities have been linked thereto. For example, heparin has been bound to artificial surfaces to reduce cooagulation by activating variuous inhibitors of the intrinsic clotting system (Salzman et al. (1987) in Hemostasis and Thrombosis: Basic Principles and Clinical Practice. 2nd Ed., (Colman et al., eds.), Lippincott Co., Phila., Pa., pp. 1335-1347). However, heparin enhances platelet responses to stimuli such as ADP or collagen, and promotes two adverse primary blood responses towards synthetic surfaces: platelet adhesion and aggregation. In addition, although surface-bound heparin/antithrombin complex may be passive towards platelets, the wide variety of effects it has on interactions with endothelial cell growth factor, inhibition of smooth muscle proliferation, and activation of lipoprotein lipase raises questions as to what adverse effects it may induce over time.
Anti-platelet agents such as PGE.sub.1, PGI.sub.2 (experimental use only), cyclic AMP, and aspirin have also been attached to solid polymer surfaces. These agents discourage the release of platelet factors that stimulate adverse healing responses in the vicinity of a vascular graft. They may also reduce platelet-aided thrombus formation by inhibiting platelet adhesion.
The exposure of many artificial surfaces to albumin prior to vascular contact results in reduced reactivity with platelets (NIH Publication No. 85-2185, Sept., 1985, pp. 19-63). Therefore, albumin has been used to coat extracorporeal surfaces before cardiopulmonary by-pass surgery. However, long-term thermoresistance has not been achieved by this procedure.
Fibrinolytically active streptokinase and urokinase, alone or in combination with heparin have been attached to artificial surfaces by Kusserow et al (Trans. Am. Soc. Artif. Intern. Organs (1971) 17:1). These enzymes reduce excessive fibrin deposition and/or thrombotic occlusions. However, the long term assessment of their ability to confer thromboresistance to a synthetic surface has not been determined.
Surface active agents such as Pluronic F-68 have also been immobilized on artificial surfaces, but do not appear to offer long term blood compatibility (Salyer et al. (1971) Medical Applications of Plastics. Biomed. Materials Res. Sym. (Gregor, ed.) No. 1 pp. 105).
Therefore, what is needed are better biocompatible materials which are thromboresistant in the long term and whose active components do not cause detrimental side affects.
An object of the present invention is to provide a synthetic, biocompatible, thromboresistent material useful for implantable and extracorporeal devices in contact with bodily fluids.
Another object is to provide an immobilized thrombogenesis inhibitor which is biologically active, and a method of preparing the same.
Still another object of this invention is to provide a method of inhibiting platelet aggregation, the release of platelet factors, and thrombogenesis at the localized site of the graft or prosthesis-blood interface, thus avoiding the systemic effect of antiplatelet and antithrombosis drugs.